Able moves forward to stand directly behind Prythurkan. With the noise and his paranoia he expected to find trouble. Reaching past the Dragonborn, Able summons a blast of eldritch power toward the nearest...creature [green]. His spear is clutched in his hand so he is ready for when they draw near.

Since the enemy hasn't gone yet, I don't know if we have advantage or anything against them - rolling it just in case it applies.

The monster stopped moving. You can decide if that means it is "dead" or not.

I've continued to refer to them in neutral terms because you definitely do not know what it is you're fighting in the dark. While you can see them as if you were in dim light, that means you're at disadvantage on Perception checks.

Unfortunately, 5e doesn't have hard and fast rules on using knowledge checks in order to identify traits about monsters. Pg 177-178 in the PHB offer this:

I periodically change my mind on what sort of action this would take. For now, I would allow the use of a Bonus Action to make an appropriate skill check to learn tidbits about these monsters. Thus far, you know they live in plant matter, erupt in pollen, shoot needles, and look like plants. I'll let you choose which Knowledge skill is the natural choice.

I'm also willing to listen to arguments about why the rule should go a different way.

All of that makes perfect sense. I'll stick with Eryndir using his bonus action to move one additional square and hide, and if he doesn't provoke an attack of opportunity, he'll draw a conclusion about what going immobile and inert means, at least as far as managing active threats goes.

In 5e, characters can move before and after they attack, so Eryndir is fine to move in this way. His total movement takes up 35 feet, due to the difficult terrain in J3 and K3, which is exactly a Wood Elf's base move. He spends his standard on the attack, then he uses his Rogue 2 ability Cunning Action:

Starting at 2nd level, your quick thinking and agility allow you to move and act quickly. You can take a bonus action on each of your turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.

He uses his cunning action to hide in the thick brown growth (although he doesn't know it is brown because his darkvision doesn't show him colors). He is able to hide in this growth because of his Wood Elf racial Mask of the Wild:

You can attempt to hide even when you are only lightly obscured by foliage, heavy rain, falling snow, mist, and other natural phenomena.

Cor'val with move straight forward to E4 (ignoring difficult terrain due to Natural Explorer feature), drawing his bow as he moves. He'll then fire on the guy in M4 (green?), with advantage (also granted from Natural Explorer). 19 arrows remaining.

Also, questions regarding 5E combat:

A) Eryndir moved around the plant he was fighting; would that provoke an AoE, or not because he stayed within the plant's reach the whole time? Is it not by the individual square you start in, but the creature's entire reach radius?

2) Are there modifiers for firing into melee? Only entry I found said you'd have disadvantage on rolls if you were yourself threatened within 5ft.

3) I take it there isn't a bonus for flanking anymore, except where noted by specific abilities? I see the sneak attack trait depends on advantage on rolls or flanking, and I've noticed various other entries (such as a wolf's pack tactics ability). There's not a general bonus or modifier for flanking though, I gather?

Not a bad entrance to a campaign! A crit is just the die roll again, not the modifier. UA ranger is just such a great class.

You're right, you ignore the difficult terrain.

M4 is a weird orange/brown color. I'm not color blind but I don't know what shade that's supposed to be. Maybe a salmon?

Question A) Here's the text: You can make an opportunity Attack when a hostile creature that you can see moves out of your reach. This has been clarified to voluntary movement. You can't shove someone out of your buddy's range so that your buddy can get a free attack. Eryndir didn't move outside the monster's reach until it was inert, so he didn't provoke.

Question B) There is no difficulty shooting into melee, only shooting out of the melee you're currently in. You can freely draw/stow weapons on your turn, so engaging an archer doesn't just ruin their day.

Question C) We use the optional flanking rules from the DMG on page 251. In short, if you're flanking, you have advantage on your attack. Flanking is a VERY good tactic.

The Kenku named Bird makes a noise that reminds everyone of a roaring bonfire. He lifts up a curious glass jar that emits bright light for 30 feet and dim light for another 30 feet beyond the limit of the bright light. Bright light extends to column G, and dim light extends to column M.

This reveals the front line monsters who are rendered inert. They appear to be man sized beings that closely resemble ghille suits or animate shrubs.

Charn gets his shield and sword ready as he goes to I3, ready to protect Eryndir with his shield in case any of the plant critters try to avenge their recently stiff comrade. He readies an attack action if possible to swat any plant critters that decide to get into melee range with him.

Caleb will cast bane on enemies #1-3, if all three are within the 30' of dim illumination cast by Bird's lightsource. If not, the spell will target as many of them as possible. He shouts, "War is the health of the State!"

No thanks. His relatively high AC may be sufficient to insulate the people in the back who haven't moved up and may be disinclined to do so, but he wants to remain as close as possible to the action without stepping out of the light himself.

While I'm sure the cleric can actually do something helpful, Able feels compelled to attempt to help Charn. I'm going to roll a d20 for a heal check - not sure what to add to it (if anything).

IC:
Seeing Charn drop, Able moves over, dropping to his knees and raising his cloak like a shield. Hopefully hidden from sight will prevent him from being attacked again. Able looks for any way to help save the dying warrior.

That's right. You don't have to dump them all into Charn, you could spread them around. However, given his high AC and overall tankyness, and the fack that he was a bloody mess a second ago, dumping them all into Charn is a great idea.

The downside is that I totally misread initiative. It isn't Caleb's turn. It should go like this.

17 Purple
16 Eryndir
13 Cor'val
11 Bird
11 Blue
8 Charn

So Purple doesn't do anything, and it is Eryndir's turn. I'm the worst. Happy new year.

Cor'val is going to move forward to J4 and cast Hail of Thorns as a bonus action. He'll then fire at what looks like #7, in Q2.

I imagine Hail of Thorns causing the shaft of his arrow growing thorns like a rose stem mid flight, and if the attack hits, thorns like needles manifesting in the air above his targets and raining down on them like...well, a hail of thorns.

My spell dc against the enemies' Dex save for half of the spell dmg is 13, and the goal is to hit enemies 6, 7, 8, and the big guy in the back.

Cor'val whispers a few words that hum with mystic might, but his arrow doesn't hit its mark. His next one, however, is going to be quite problematic for its quarry.

Bird advances: B3, C2, D2, E2, F2, G2. He opens his beak and creates the sound of a crackling campfire. The Kenku reaches into a bag at his waist and pulls out several vials - the bird brought alchemist's fire and has more than enough for everybody.